When producing BHJ OPV cells it is important that the separation of donor and acceptor in the active layer is optimised to enable high power conversion efficiency. Typically a mixture of donor and acceptor are advantageously coated from a solvent, and phase separation then occurs during the drying process. A number of different coating techniques are possible, like spin coating, blade coating, etc. Blade coating is preferentially used due the ease of transfer to larger industrial coating processes (see F. Padinger, C. J. Brabec, T. Fromherz, J. C. Hummelen, N. S. Sariciftci, Opto-Electron. Rev. 2000, 8, 280), and because of its comparability to spin coating (see P. Schilinsky, C. Waldauf, C. J. Brabec, Adv. Funct. Mater., 2006, 16, 1669). The coating technique may also have an impact on the phase separation, but this usually tends to be of less relevance compared to the physical parameters of the materials and solvents involved in the process.
Typical donor/acceptor blends described in prior art include as a donor polymer for example poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) or a-PTPTBT as shown below,
and as acceptor for example a C60 or C70 fullerene or a substituted C60 or C70 fullerene, like for example C60PCBM as shown below or C70PCBM.

Due to the limited solubility especially of the acceptor component, the donor/acceptor blends tend to be processed from solutions of chlorinated solvent such as chlorobenzene, trichlorobenzene, dichlorobenzene or chloroform, and the majority of the literature gives examples of P3HT and C60/C70 blends coated from o-dichlorobenzene (DCB) or chlorobenzene.
The morphology and phase separation of this P3HT/PCBM system has been extensively studied by AFM techniques, and it has been shown that forming the correct morphology is critical to enable efficient charge separation and collection in the cell (see Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. Adv. Mater. 2009, 21, 1521-1527). It has also been stated that the phase diagram for the two components is likely to be unique for each donor/acceptor system and to-date remains to be fully described for any one system, making obtaining the correct morphology in new blends a major challenge (see C. Muller, T. A. M. Ferenczi, M. Campoy-Guiles, J. M. Frost, B. B. C. Bradley, P. Smith, N. Stingelin-Stutzmann, J. Nelson, Adv. Mater. 2008, 20, 3510, and B. Lei, Y. Yao, A. Kumar, Y. Yang, V. Ozolins, J. Appl. Phys. 2008, 104, 024504).
In the drive to further improve the power conversion efficiency (PCE) of OPV devices, it is desirable that the morphology of the donor and acceptor blend, which forms the heterojunction responsible for the transport of charge in the OPV device, can be intentionally manipulated. The literature illustrates many examples of ways in which this has been attempted, for example by modification of the solvent and solvent blends used in preparation of the active layer. A good overview of the area is given in the paper by Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. Adv. Mater. 2009, 21, 1521-1527.
In BHJ OPV devices the choice of the solvent, the polymers used and the deposition conditions/techniques determine the phase separation morphology between the donor polymer and the acceptor, usually PCBM. High PCE has been obtained in the literature by optimizing the blends and coating conditions from donor/acceptor formulations mainly using chlorinated solvents.
However, when moving from research to industrial processes, chlorinated solvents are not preferred, and do therefore need to be replaced by solvents that are more suitable for large scale coating applications. The aim is therefore to use a non-chlorinated solvent while still retaining the high PCE.
Currently, there is limited literature regarding the use of non-chlorinated solvents for forming BHJ OPV devices. In some cases non-chlorinated solvents are used together in a blend with chlorinated solvents, (see C. Hoth et al., J. Mater. Chem. 2009, 19, 5398, F. Zhang et al., Adv. Funct. Mater. 2006, 16, 667) but only a few examples of high efficiency OPV devices were reported. In some cases, data obtained using solutions of P3HT/PCBM in alternative solvents such as toluene, xylene, cyclohexanone or tetralin are shown, but compared to chlorinated solvents these generally show worse performance. (see C. Lin et al., Synth. Metals. 2010, 160, 2643, Pure and Applied Chem. 2008, 80, 2151, S.-R. Tseng et al., Organic Electronics, 2009, 10, 74, M. Rispens et al., Chem. Com., 2003, 2116, T. Aernouts et al., Appl. Phys. Lett. 2008, 92, 033306, S. E. Shaheen et al., Appl. Phys. Lett. 2001, 78, 841, J. Liu et al., Adv. Funct. Mater. 2001, 11, 420) This has been attributed to the poor solubility of both the P3HT and the PCBM in these alternative solvents, which results in rough films containing large crystallites of PCBM which disrupt the morphology (see Nilsson, S.; Bernasik, A.; Budkowski, A.; Moons, E. Macromolecules 2007, 40, 8291, and Chan, S.; Hsiao, Y.; Hung, L.; Hwang, G.; Chen, H.; Ting, C; Chen, C. Macromolecules, 2010, 43, 3399).
US 2010/0043876 A1 discloses a composition for forming an active layer in an OPV device comprising a p-type material like for example P3HT, an n-type material like for example a C60 fullerene, a first solvent which comprises at least one alkylbenzene (like xylene or toluene) or benzocyclohexane (i.e. tetralin), and a second solvent which is different from the first solvent and comprises at least one carbocyclic compound. It is further disclosed that the solvents can be selected according to their Hansen Solubility Parameters. The boiling point of the first solvent can be greater than that of the second solvent or vice versa. The exemplified solvent systems include o-xylene/tetralin, toluene/salicylaldehyde, o-xylenetsalicylaldehyde, o-xylene/tetralin/salicylaldehyde, toluene/methyl salicylate, toluene/anisole, tetralin/toluene/salicylaldehyde, and tetralin/toluene/anisaldehyde.
However, the non-chlorinated solvent systems as disclosed in prior art do still have several drawbacks and leave room for further improvement. For example, regarding the solvent systems disclosed in US 2010/0043876 A1, the P3HT used therein is known to have a limited solubility in various organic solvents, and therefore there is a limited scope for solvent manipulation to influence phase separation. US 2010/0043876 A1 describes how adding a second solvent which constitutes a small percentage of the ink composition can modify the efficiency of the device. The second solvent is generally a high boiling solvent, and has poor solubility for the P3HT component, causing it to crystallise out of solution more readily and hence tending to a rougher active layer, as shown by AFM imaging.
A small amount of a solvent with poor solubility for one of the components has the effect of increasing the roughness, which implies that there is an increase in the size of the crystals/grain boundarys formed in the P3HT or PCBM. One possible reason for very rough films is the formation of crystallites which are increasing in size and can lead to large PCBM or P3HT islands, and therefore does not give the optimum phase separated morphology and performance.
In US 2010/0043876 A1 the addition of a second solvent is linked with the increase in roughness and the increased performance in a system containing P3HT. However, as will be shown in the present invention, drawing a universal link between surface roughness and performance is not always valid. The examples of the present invention demonstrate that it is also possible to improve device performance even if there is no significant change in the surface roughness of the active layer.
It is desirable to provide solvent systems that increase the performance but do not necessarily also increase the roughness of the active layer to avoids potential issues when further layers are deposited on top of the active layer.
In addition, the polymer utilised in the embodiments presented in US 2010/0043876 A1 is P3HT, which is a special case in the field of OPV due to its semi-crystalline/crystalline nature, whereas recently developed low band-gap polymers, which are required for higher efficiencies, are typically more amorphous in character. Therefore improved solvent systems are required that are specifically developed for these new polymers.
Also, the solvents described in US 2010/0043876 A1, which are used as replacement for chlorinated solvents such as DCB, appear to have disadvantages compared to DCB regarding device performance.
Thus, in the examples disclosed in US 2010/0043876 A1, P3HT is formulated with C-60 indene using a range of non-chlorinated solvent mixtures with varying composition. A comparison device prepared by using using DCB as solvent is not disclosed therein. However, it has been shown in literature that when P3HT/C-60 indene C60 is coated from DCB, an average of 5.44% PCE can be achieved (see Y. He et al., J. Am. Chem. Soc, 2010, 132, 1377). In contrast thereto, when P3HT/C-60 indene is coated from toluene as exemplified in US 2010/0043876 A1 as a single solvent in replacement for DCB, this shows a decreased PCE of 3.73%. The use of dual/terniary solvent mixtures as further exemplified in US 2010/0043876 A1 shows that the performance of P3HT/C-60 indene can be improved, however, neither of the formulations exemplified can reach or exceed the performance obtained when using DCB as solvent.
In conclusion, the approach as described in US 2010/0043876 A1 to replace DCB by alternative solvents is not definitive, is limited to a specific material, and appears to be of limited benefit considering the PCE values achieved.
Therefore, there is still a need for improved solvent systems and OSC formulations which can be used as OSC inks for the manufacture of OE devices, especially OFETs and OPV cells, which allow a broad, but precise selection of solvents that have suitable viscosity, are non-chlorinated and do not adversely affect the properties and performance of the OE device and its components, like the roughness of the active layer or the PCE of the device.
One aim of the present invention is to provide solvent systems and OSC formulations having these advantages. Another aim is to provide improved OE/OPV devices obtainable from such OSC formulations. Further aims are immediately evident to the person skilled in the art from the following description.
The inventors of the present invention have found these aims can be achieved, and the above-mentioned problems can be solved, by providing solvent systems and OSC formulations as claimed in the present invention.
In particular, the inventors of the present invention have found that an OSC formulation can be prepared utilising solvent mixtures to affect the phase separation of the two components. This was confirmed by observing a difference in performance between OE devices prepared from the same OSC material in different solvent systems, when all remaining factors are kept constant and only the formulation composition is varied.